Multiscale and Rough Skin/Iron Barbs are my two favorite abilities so far, mostly thanks to alleviating pressure on our stats. Taking a look at the VR, Water/Fairy is a typing that wants to be able to hit decently hard with its STAB moves. That is, being able to score 2HKOs with super effective hits on the Pokemon it can switch into is very valuable for a wall, but it presents an interesting challenge: stats will get spread a bit thin trying to have the bulk, power, and speed required for this concept.
Multiscale helps CAP33 wall Pokemon by boosting its bulk as long as it can keep its HP up. Between its stat spread and 8 PP recovery moves (or alternate recovery options), I think this would make for a Pokemon that takes some decent skill to pilot; however, Multiscale would enable the gameplan of a fast wall extremely solidly. We saw how bulky Defog + Multiscale Dragonite worked last generation despite its 4x weakness to Ice and unstabbed coverage. I'm confident that a optimized stat spread, typing, and movesets will make CAP33 a solid user of Multiscale as well.
Rough Skin/Iron Barbs, on the other hand, helps CAP33 shore up its damage output, applying pressure just by switching in and making damage output from STAB moves less critical. This reminds me of Rocky Helmet Tornadus-T applying pressure to Kartana in Gen 7 and forcing it out the following turns. Water/Fairy is a fantastic typing to have Rough Skin as well, having a laundry list of very positive interactions in the S/A ranks on the current viability rankings alone:
: CAP33 is immune to otherwise strong non-contact Dragon Darts, and it resists U-turn, potentially punishing basically any Dragapult set that wants to scout.
: CAP33 should be able to switch into Kowtow Cleave easily and Iron Head decently, forcing chip damage.
: Both Knock Off and Circle Throw make contact, making CAP33 an even better switch-in.
: CAP33 can punish Equilibra's Rapid Spin and well as pressure it with a STAB Water-type move (barring Tera).
: Same logic as Equilibra.
: A very chipped Hemogoblin might be forced to KO itself from the Rough Skin/Iron Barbs damage, as using another move may not be possible due to Moonblast.
: Gets chipped if CAP33 switches into U-turn; though CAP33 has to watch out for Gunk Shot.
: Same logic as Hemogoblin, but it's pressure by STAB Fairy-type attacks instead (again, barring Tera).
: Any set utilizing Close Combat or Knock Off, which is quite a few, will risk taking chip.
:Hisuian Samurott:: Basically all of its moves will activate Rough Skin/Iron Barbs.
: All of its moves make contact. Wild Charge is its only move that threatens CAP33, which is usually dropped in favor of Psychic Fangs and still makes contact.
: Very similar reasoning to Dragapult.
This list doesn't include interactions that could be possible by Terastallizing, which could capture improved matchups from Sneasler and Venomicon (Body Press or Knock Off), as well as more positive matchups from the B/C ranks. However, it should be obvious that Water/Fairy's resistance to Dark, Bug, and Fighting (and immunity to Dragon for the Dragapult/Roaring Moon matchups) enables CAP33 to really dish out chip damage. Notably, Baxcalibur and Walking Wake avoid Rough Skin/Iron Barbs; however, CAP33 already has a positive matchup into them by typing alone.
The reason why I like Rough Skin/Iron Barbs as opposed to other contact-punishing abilities like Flame Body and Static is that Rough Skin/Iron Barbs doesn't force CAP33 to commit to certain paths. Sure, Rough Skin/Iron Barbs might imply CAP33 running Rocky Helmet some of the time, but it leaves the door open to CAP33 running any status-inducing move it wants to. It's also extremely consistent - you know that switching CAP33 into a contact move will always deal the chip damage, rather than hoping for a 30% chance to activate.
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A couple honorary mentions:
Shield Dust turns the Garganacl matchup around, prevents CAP33 from being overwhelmed by drops from Dragapult's Shadow Ball or Hoopa's Psychic or being flinched by Baxcalibur's Icicle Crash. However, CAP33's typing means that one of the biggest benefits of Shield Dust - eating Sludge Bombs or Sneasler's Dire Claw without fearing the poison/status chance - is not be leveraged. I don't feel comfortable with how Shield Dust's main advantage is the Garganacl matchup, which is not drastically improving CAP33's ability to be a fast wall.
Serene Grace has some interesting interactions with STAB moves - some of which would help with the stat spread issue I mentioned before - but I still need to think about how this ability really interacts with everything.